JDK-8145371 ClassCastException thrown in LambdaFormEditor.getInCache

[Martin Buchholz]

Here at Google we tried to fix JDK-8145371 because it looked like it was
causing rare problems in production.  But after looking at the code, I'm
not sure...  Anyways,

http://cr.openjdk.java.net/~martin/webrevs/jdk/MethodHandle.form/
https://bugs.openjdk.java.net/browse/JDK-8145371
Copying to a local in customize() must be an improvement.
The UNSAFE publication in updateForm looks fishy, as does that comment
in MethodHandleImpl.java.  Is there something the fullFence() is supposed
to do that isn't taken care of by putObjectVolatile ?

Feel free to take ownership of this bug from me.

--- a/src/java.base/share/classes/java/lang/invoke/MethodHandle.java
+++ b/src/java.base/share/classes/java/lang/invoke/MethodHandle.java
@@ -1660,13 +1660,13 @@
         assert(newForm.customized == null || newForm.customized == this);
         if (form == newForm)  return;
         newForm.prepare();  // as in MethodHandle.<init>
-        UNSAFE.putObject(this, FORM_OFFSET, newForm);
-        UNSAFE.fullFence();
+        UNSAFE.putObjectVolatile(this, FORM_OFFSET, newForm);
     }

     /** Craft a LambdaForm customized for this particular MethodHandle */
     /*non-public*/
     void customize() {
+        final LambdaForm form = this.form;
         if (form.customized == null) {
             LambdaForm newForm = form.customize(this);
             updateForm(newForm);
diff --git a/src/java.base/share/classes/java/lang/invoke/MethodHandleImpl.java
b/src/java.base/share/classes/java/lang/invoke/MethodHandleImpl.java
--- a/src/java.base/share/classes/java/lang/invoke/MethodHandleImpl.java
+++ b/src/java.base/share/classes/java/lang/invoke/MethodHandleImpl.java
@@ -909,7 +909,7 @@
             int c = count;
             maybeCustomizeTarget();
             if (c <= 1) {
-                // Try to limit number of updates.
MethodHandle.updateForm() doesn't guarantee LF update visibility.
+                // Try to limit number of updates.
                 if (isCounting) {
                     isCounting = false;
                     return true;